This patch defers timing updates from the Web Animations API to
animation frames. Updating timing will now request an animation
frame and update the timing information on that frame. We store
bits on the Timelines and Players to indicate whether they need
updates.

The timing information on a TimedItem is updated upon access if
its Player needs an update. The bit on Timeline is used to make
sure getComputedStyle returns up-to-date information.

BUG=334926

Review URL: https://codereview.chromium.org/159403002

git-svn-id: svn://svn.chromium.org/blink/trunk@166977 bbb929c8-8fbe-4397-9dbb-9b2b20218538